1. Field of the Invention
The present invention relates to the preparation of "polyphenols", i.e., phenolic compounds containing at least two nuclear hydroxyl groups and optionally one aldehyde group, and, more especially, to the preparation of such polyphenols by oxidation of hydroxybenzaldehydes with hydrogen peroxide.
2. Description of the Prior Art
Polyphenols, whether unsubstituted or substituted with functional groups such as aldehyde and/or alkoxy groups, are in wide demand as industrial products. Thus, hydroquinone is widely used, in particularly, in the photographic industry. Pyrocatechol is in turn a particularly important raw material for the preparation of certain organic compounds, such as guaiacol and its derivatives, or for the preparation of resins by condensation with formaldehyde. Pyrogallol is also employed as a photographic developer and as an intermediate in various organic syntheses. Likewise as regards protocatechaualdehyde (3,4-dihydroxybenzaldehyde), hydroxy-para-vanillin (3,4-dihydroxy-5-methoxybenzaldehyde) and hydroxy-ortho-vanillin (2,5-dihydroxy-3-methoxybenzaldehyde).
And numerous processes for the preparation of polyphenols have to date been proposed to this art. Among such processes, two types in particular have in common the oxidation of a phenolic compound with hydrogen peroxide and the peroxides derived therefrom, and especially inorganic or organic peracids.
The aforesaid two types of processes differ in the nature of the starting materials. Thus, it is known to prepare polyphenols by direct hydroxylation of the aromatic nucleus with hydrogen peroxide or organic peracids such as performic and peracetic acids. Hydroxylation processes of this type, which are especially suited for the preparation of hydroquinone and pyrocatechol have been described, in particular, in U.S. Pat. Nos. 3,514,490, 3,849,502 and 4,208,536. Although these processes have proven to be of great value, it is advantageous in certain instances to use a second group of processes, the characteristic of which is the substitution of one or more aldehyde groups of a benzaldehyde by one or more hydroxyl groups.
The oxidation of aromatic aldehydes to the corresponding phenols by means of hydrogen peroxide or percarboxylic acids, which is generally known as the Bayer and Villiger reaction, is a convenient means of obtaining a phenol, if a source of aromatic aldehydes is available (compare C. H. Hassal, Organic Reactions, Volume 9, pages 73 to 106 (1957)). The Bayer and Villiger reaction actually entails two types of reaction. The first consists of the oxidation of aromatic aldehydes with percarboxylic acids formed "in situ" or prepared for immediate use by reacting hydrogen peroxide with a carboxylic acid such as formic, acetic or benzoic acids. In all cases, this oxidation can be carried out in the presence of a strong acid as a catalyst (for example, toluenesulfonic acid). Not all aromatic aldehydes yield phenols according to this reaction. In fact, it has been found that benzaldehyde and its homologs containing electron-attracting substituents (for example, halogen atoms or the nitro group) are oxidized by peracids to the corresponding benzoic acids, while aromatic aldehydes containing electron-donating substituents (hydroxyl, alkoxy or alkyl groups), such as salicylaldehyde, p-hydroxybenzaldehyde and ortho- and p-methoxybenzaldehydes, give rise to the corresponding phenols (if appropriate in the form of formates, depending on the reaction conditions) (compare J. Boeseken et al, Rec. Trav. Chim. Pays-Bas, 55, 815 (1936); J. Boeseken et al, Ibid., 74, 845 (1941); and Y. Ogata et al, J. Org. Chem., 26, 4,803 (1961)). This process of oxidation of aromatic aldehydes containing electron-donating substituents to phenols exhibits the disadvantage in that it requires the use of a carboxylic acid, which in fact acts as an active-oxygen carrier in the reaction. The presence of the carboxylic acid in the process mandates the use of large reaction volumes, which limits the productivity of the reaction on an industrial scale. Furthermore, the fact that the reaction is carried out in an anhydrous medium, in order to assist the formation of the peracids, creates explosion hazards. It is thus desirable to dispense with the use of any carboxylic acid and consequently to use hydrogen peroxide directly as the source of active oxygen. This object is achieved by carrying out the reaction in an alkaline medium, and this constitutes the second group of processes classed as the Bayer and Villiger reaction, and generally designated as the Dakin reaction. According to Dakin (compare Amer. Chem. J., 42, 474 (1909)), aromatic aldehydes containing one or more hydroxyl groups in the ortho- or para-position to the carbonyl group are oxidized in good yields to the corresponding polyphenols by hydrogen peroxide in an alkaline medium. Under these conditions, the aldehyde groups in the meta-position to the phenolic hydroxyl are not oxidized. This process has been applied to numerous aromatic aldehydes containing at least one hydroxyl group in the ortho- or para-position, such as, in particular, salicylaldehyde, p-hydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2-hydroxy-3-methoxybenzaldehyde, 2-hydroxy-5-methoxybenzaldehyde and 3-hydroxy-5-methoxybenzaldehyde. Distinct from the Bayer and Villiger reaction itself, the Dakin reaction applies to hydroxybenzaldehydes substituted by halogen atoms, such as 3,5-dibromo-4-hydroxybenzaldehyde, 3,5-dichloro-4-hydroxybenzaldehyde, 5-bromo-2-hydroxybenzaldehyde and 3-bromo-4-hydroxy-5-methoxybenzaldehyde, and to certain nitrohydroxybenzaldehydes, such as 3-nitro-2-hydroxybenzaldehyde, 5-nitro-2-hydroxybenzaldehyde and 2-nitro-4-hydroxy-3-methoxybenzaldehyde (compare Dakin loc. cit.; Dakin, Org. Synth. Coll., Volume 1, page 149 (1941); Surrey, Org. Synth., 26, page 90 (1946); and J. Kvalnes, J. Amer. Chem. Soc., 56, 2,487 (1934)).
The Dakin reaction is carried out in an aqueous solution of an alkali metal base, such as sodium hydroxide, the pH of the medium being strongly alkaline and characteristically, above 8. Under these conditions, it is impossible to avoid undesirable oxidation of the reaction products to quinones. Furthermore, it has been found that, in the case of hydroxybenzaldehydes containing both an aldehyde group in the para-position and at least one aldehyde group in the ortho-position to the phenolic hydroxyl, the yields of products resulting only from the oxidation of the aldehyde groups in the ortho-position are low in an alkaline medium, even if the amount of hydrogen peroxide is insufficient to oxidize all the aldehyde groups present. It is therefore impossible, under the usual conditions of the Dakin reaction, to prepare polyhydroxybenzaldehydes with yields high enought to be of industrial value by oxidizing only the aldehyde groups in the ortho-position to the phenolic hydroxyl. This is the case as regards the oxidation of 2-hydroxy-3-methoxyisophthaldehyde (diformylguaiacol) to ortho-hydroxy-p-vanillin. In the same way, it can prove advantageous on an industrial scale to be able to oxidize, with good yields, a mixture of an ortho-hydroxybenzaldehyde and a para-hydroxybenzaldehyde, such as a mixture of salicylaldehyde and p-hydroxybenzaldehyde, or a mixture of vanillin and 2-hydroxy-3-methoxybenzaldehyde.